Fasciola hepatica is a parasite causing a liver disease in both man and cattle in Puerto Rico. This increase in this parasitism warrants investigations aimed at understanding this parasite cycle and introducing measures of control and prevention. The proposed investigations are directed towards accomplishing these goals through the following: To analyze on a regional basis the known prevalence of F. hepatica in cattle across the island. To determine the prevalence of F. hepatica in P.R. in humans and goats. To locate the possible sources of human infection of F. hepatica. To determine the ecological factors that contribute to the spread and incidence of F. hepatica on the farms. To determine the effects of concurrent infections of F. hepatica with other parasites commonly found in cattle such as Cotylophoron cotylophorum and Haemonchus contortus as a means of biological control. Molluscicidal assay of several plants revealed Solanaceae to be toxic to L. cubensis snails. The water and the methanol extracts of S. mammosum fruits were toxic for 5 percent and 95 percent of the snail population at 25 ppm. Analysis of the methanolic extract showed the glycosidic steroid alkaloids solasonine and solamargine to be the active products, 100 percent mortality of snails at 10 ppm. Preliminary structure-activity relationship studies suggested a class of glycosidic steroids to be molluscicidal to various parasites transmitting snail species. Other structurally simple amines were shown to be less active.